The present invention relates to a developing apparatus to be installed in an electrophotographic copying machine, a developing method, and an image forming apparatus.
In the developing apparatus of this type, a two-component developing agent made of a carrier and a toner is fed to a developing roller while stirring and circulating it with the aid of rotation of the feed mixer. The developing agent fed to the developing roller is further supplied to an electrostatic latent image formed on a photosensitive drum by the aid of rotation of the developing roller. In this way, the latent image is developed.
The feed mixer of the developing apparatus has a shaft member which has a spiral-form blade body for feeding the developing agent in a shaft direction. To the shaft member of the feed mixer, a paddle is horizontally fitted for scraping the developing agent upwardly to the position at which the developing roller is located.
The developing apparatuses includes a face-side type and a face-up type. In the face-side type, a developing roller is placed at a side of the photosensitive drum so as to face a side surface portion of the photosensitive drum. In the face-up type, the developing roller is placed under the photosensitive drum so as to face a lower surface of the photosensitive drum.
In the case of the face-up type developing apparatus, since the photosensitive drum is placed above the developing apparatus, the fixing position of a doctor blade is lowered in such a manner that the doctor blade for regulating a thickness of a layer of the developing agent formed on the developing roller is not in contact with the photosensitive drum. With this arrangement, the fixing position of the feed mixer is lowered. As the result, the position of the developing roller is placed higher than a center shaft of the feed mixer. Therefore, in the case of the face-up developing apparatus, it is necessary to increase the function of the feed mixer for scraping the developing agent upward, compared to the face-side developing apparatus.
Then, in a conventional type, the area of the paddle is enlarged by increasing a width of the paddle to improve the scraping-up function of the paddle.
However, if the scraping-up function of the feed mixer is simply increased, the ability for feeding the developing agent in a shaft direction is lowered, with the result that a speed for circulating the developing agent decreases. If the circulation speed decreases, the image is formed non-uniformly in concentration particularly in the case where a copy speed is high or where a printing rate is high.